Melting furnaces and vessels can be used to melt a wide variety of batch materials, such as glass and metal batch materials, to name a few. Batch materials can be placed in a vessel having two or more electrodes and melted by applying voltage to the electrodes and/or by applying an external heat source, such as a burner. The life cycle of a melting furnace can depend, e.g., on wear of the refractory materials from which the vessel is constructed. For instance, during the melting process, the vessel walls can be gradually worn down due to contact with the molten batch materials. Refractory materials used to construct the walls, bottom, and/or top of the vessel should thus exhibit high corrosion resistance, low thermal conductivity, high electric resistivity, and/or high mechanical strength to survive the rigorous temperatures and other conditions associated with processing molten materials.
Wear of the refractory material during the melting process not only poses a safety risk in terms of creating leakage pathways that can comprise the operational safety of the equipment, but can also contaminate the batch materials. For instance, if a piece of the refractory wall breaks off into the melt, it may result in an unacceptable impurity or inclusion defect in the final product. It can therefore be important to manufacture refractory materials, such as zirconia, for the construction of melting vessels and other high-temperature equipment that can withstand processing rigors for extended periods of time without compromising operational safety and/or product quality. However, refractory blocks used to manufacture such melting vessels can be relatively large (e.g., about 20-25 cm thick, about 30-60 cm wide, about 120-140 cm long) and/or heavy (e.g., about 300-400 kg for a 20 cm×30 cm×120 cm block or about 1000-1100 kg for a 25 cm×60 cm×140 cm block), which can complicate the manufacturing process from a scalability and/or process management standpoint.
One conventional process for forming refractory blocks is fused casting, in which batch materials are melted (e.g., in an arc furnace with graphite electrodes) and the melt is poured into a crucible (e.g., a graphite crucible), followed by a controlled cooling cycle. Refractories produced by such processes can be exposed to reducing atmospheres (e.g., due to graphite electrodes and/or crucibles), which may result in reduced species in the material such as graphite, zirconium carbide, and/or zirconia metal, and/or voids due to oxygen vacancies in the material. Reduced species within the refractory can result in a refractory product that is substantially gray in color, but which can turn to a cream to light brownish color as the refractory changes oxidation states during use.
Fused cast high-zirconia refractories can be expensive to manufacture and/or can have various drawbacks such as stoning and/or significant yield loss, particularly within the first few months of process start-up. During the cooling cycle used in the fuse casting process, voids and/or porosity can form at the center of the refractory material due to organic burnoff and/or reduction reactions. Thus, there may be variations in the crystal size and/or glassy composition across the thickness of the material and/or at the surface as compared to the center. In some instances, the grain size can increase from relatively smaller at the surface to relatively larger at the center. These fused cast refractories, such as zirconia, can also comprise fairly large grains (e.g., about 100-1000 microns), which may pose a problem if they detach from the surface. Larger zirconia grains or pieces of refractory can break off into the melt and result in an impurity and/or inclusion defect in the final product.
Currently, to Applicant's knowledge, sintered high-zirconia refractory materials suitable for constructing melting vessels have not been contemplated or made available. This may be due, at least in part, to various difficulties in managing the manufacturing process, such as phase changes brought on by elevated temperatures, which can cause stress and/or significant volume changes within the refractory material. For example, zirconia can undergo a change in crystal structure around 1170° C. from a monoclinic to a tetragonal structure. Such crystal structure changes can be associated with significant volume changes (e.g., as high as about 4%), which can make it difficult to manage the manufacturing process, particularly for large-scale applications, and/or can add stress to the refractory parts during use at elevated temperatures. Bonded (sintered) alumina-zirconia-silica (AZS) refractories may be available as materials for constructing melting vessels; however, such AZS materials typically comprise less than about 40 wt % zirconia in the composition, and thus exhibit lower resistance to corrosion (e.g., from molten glass and/or metal) as compared to high-zirconia refractories (e.g., greater than about 80 wt % zirconia).
Accordingly, it would be advantageous to provide refractory materials that can withstand high temperatures and/or corrosive conditions for extended periods of time without compromising safety and/or product quality. It would be also be advantageous to provide methods for producing such refractories that have reduced cost and/or complexity. Moreover, it would be advantageous to provide methods for forming large refractory shapes, such as blocks, which can have dimensions suitable for constructing large-scale equipment.